Conventionally, catalyst from a fluid catalyst cracking (FCC) unit is regenerated because it is contaminated with a carbonaceous deposit, coke, in an operating fluid bed catalytic cracker. An FCC regenerator operates at very high oxidative temperature due to the high heat release of burning coke. The capacity of many FCC units is limited by the regenerator operating temperature, which approaches the limit beyond which the regenerator may not be operable because of its metallurgy. Hot regenerated catalyst is conventionally cooled in a catalyst cooler ("catcooler") by generating steam. The catcooler may be either internal or external to the regenerator vessel.
Heat generated in a conventional regenerator is typically removed by internal coils (regen coils) functioning as an internal catcooler, or, by an ECC (external catcooler) in which hot catalyst is contacted with cooling fluid in heat exchanger tubes. Prolonged operation of a regenerator at a temperature at which the catalyst's efficiency is not deleteriously affected bestows upon the operation of any catcooler, a criticality which demands near-absolute reliability of operation. Because such reliability has been so well established by tubes carrying cooling fluid, a heat exchanger is a logical choice, and has been for many years. Because of the relatively high temperature, in the range from about 621.degree. C. to 732.degree. C. (1150.degree.-1350.degree. F.), at which a large volume of regenerated catalyst must be returned to the cracker of an operating refinery, it is evident that the drop in catalyst temperature due to cooling cannot be large, but the amount of heat to be removed is very large. This makes the generation of steam a logical choice.
Prior art FCC regenerators with catcoolers are disclosed in U.S. Pat. Nos. 2,377,935; 2,386,491; 2,662,050; 2,492,948; and 4,374,750. All these prior art catcoolers remove heat by indirect heat exchange, typically a shell and tube exchanger. None removes heat by direct heat exchange, for example, by continuously diluting hot regenerated catalyst with cold catalyst, or by blowing cold air through the hot catalyst; more particularly, none removes heat by functioning as a reactor which supplies heat to an endothermic reaction.
U.S. Pat. No. 4,422,925 discloses the step-wise introduction of ethane, propane, butane, recycle naphtha, naphtha feed, raffinate naphtha, and fractionator bottoms recycle in the riser reactors of a FCC unit. In the riser reactors, the lower alkanes are contacted, in a transport zone, with hot regenerated catalyst which would dehydrogenate the alkanes, progressively decreasing the temperature of the suspension of catalyst and hydrocarbons as they progress upwards through the risers. The mixture of catalyst and reaction products is then contacted with a hydrocarbon feedstock suitable for catalytic cracking, such as virgin naphtha, virgin gas oil, light cycle gas oil, or heavy gas oil.
The control of the catalyst temperature in the risers as well as the benefits of dehydrogenation occurring in the risers were both lost when the products from the risers were mixed with the products of the main cracker. Most important is that operation of the reference FCC unit as a combination dehydrogenator and cat cracker failed to provide control of the olefins generated, which control is essential if recovery and subsequent utilization of the olefins is a goal of the process.
The concept of cooling hot regenerated catalyst by using an endothermic reaction, specifically the catalytic dehydrogenation of butane with chromic oxide supported on alumina or magnesia, was disclosed in U.S. Pat. No. 2,397,352 (Hemminger). Though unrelated to operation of a FCC unit, regeneration of the catalyst was required before it was returned to the dehydrogenation reactor. Hemminger provided a catalyst heating chamber for supplying heat to the dehydrogenation reaction to compensate for that lost in the dehydrogenation reaction, and to preheat the butane to raise its temperature to reaction temperatures. Since the disclosure of this old process, the use of large pore zeolites for cracking catalysts was discovered, as was the effectiveness of certain large and intermediate pore zeolites for the conversion of alkanes to olefins. Our process achieves at least 50%, and preferably 70% conversion of propane, and for the first time, makes the process practical in a refinery environment. Though conceptually feasible, the '352 system required pressurizing catalyst powder which was to be recirculated. The result was that the catalyst did not recirculate.
At the present time, there exists a profusion of schemes for dehydrogenating propane, eventually, for the most part, converting it to gasoline or other products far more valuable than propane. For example, U.S Pat. No. 4,293,722 to Ward et al, teaches one such process. These schemes are unequivocally based on the catalytic effect of particular catalysts and rely on reactions which occur at substantially lower temperatures than those used in our process.
This invention relates to an improvement in cooling regen catalyst, which improvement involves novel apparatus and operating techniques for fluid bed FCC catalyst in an ECC under conditions such that the catalyst is cooled while it performs its alkane dehydrogenating function and thereafter may be returned from the ECC to the regenerator.